Process for producing naphthalene



United States Patent O 3,215,749 PROCESS FOR PRODUCING NAPHTHALENE Herbert L. Johnson, Media, and Alfred Hahn, Jr., Lansvdowue, Pa., assignors to Sun Oil Company, Philadelphia, Pa., a corporation of New Jersey Filed May 18, 1961, Ser. No. 111,005 6 Claims. (Cl. 260-672) This invention relates to the preparation of naphthalene from hydrocarbon stocks and more particularly to the dealkylation of hydrocarbon stocks which comprise alkyl naphthalenes for the purpose of conversion to naphthalene.

Petroleum fractions which boil within the range of 400-550 F. generally contain substantial amounts of alkyl naphthalenes, such as mono-, di-, and trimethylnaphthalenes and in smaller quantity, the ethyl naphthalenes. Recycle fractions, which are formed in the cracking of petroleum stocks and which include this boiling range, often contain major proportions of aromatic hydrocarbons that are mainly alklyl naphthalenes. Such fractions typically may have aromatic contents varying within the range of '25-97% but usually contain between 50% and 95% aromatics depending upon the particular operation in which the petroleum fractions are produced. These hydrocarbon charge stocks are obtained in both catalytic and thermal cracking processes and in operations in which combinations of catalytic and thermal cracking steps are utilized. Stocks having high alkyl naphthalene contents can also be obtained by extracting straight run petroleum fractions of appropriate boiling ranges, such as kerosene, or catalytic fractions such as catalytic gas oil, with solvents, such as furfural or sulfur dioxide, or by selective adsorption with silica gel. These aromatic concentrates may contain up to 100% aromatic hydrocarbons.

The present invention is directed to the preparation of naphthalene from aromatic hydrocarbon charge stocks Which comprise a mixture of alkyl naphthalenes and which can be derived from such sources as referred to above. The charge stock generally boils within the range of 400- 550 F. but more preferably in the range of 440-515 F. and preferably is composed mainly of alkyl naphthalenes in which the alkyl groups appear at both the alpha and beta positions on the naphthalene nucleus.

It has been proposed heretofore to prepare naphthalene by subjecting aromatic hydrocarbon stocks containing alkyl naphthalenes to high temperature dealkylation in the presence of hydrogen. As a general rule, the dealkylation conditions employed can effect only a partial dealkylation in one pass of the alkyl naphthalenes through the reactor. In order to increase the yield of naphthalene, it is desirable to fractionate the reaction product to obtain the desired naphthalene product and also to recover another fraction comprising unconverted and only partially converted alkyl naphthalenes. The recovered alkyl naphthalenes can be recycled to the dealkylator to effect further conversion.

It has now been found that when the alkyl naphthalenes, which are separately recovered from the dealkylator reaction product, are recycled for further reaction along With additional charge stock, the overall rate of dealkylation is substantially reduced. This is for the reason that under the dealkylation conditions employed, which generally include a temperature in the range of 1100-1500D F. and more usually 1200-l400 F., the alkyl substituents Which are in the alpha position are removed from the naphthalene nucleus at a rate which is roughly about twice as fast as the rate of removal of beta substituents. Hence the alkyl naphthalenes that are recovered after the reaction are considerably enriched "ice with respect to components having beta substituents; and this material When recycled is substantially more refractory than the original charge stock.

The present invention provides an improved process for producing naphthalene in which a substantially increased overall rate of dealkylation of alkyl naphthalenes is attained. According to the invention, the alkyl naphthalenes recovered from the dealkylation product are subjected to an isomerization operation in which the conditions are effective to shift the position of lalkyl groups on the naphthalene nucleus. The isomerization results in an alkyl naphthalene product having an increased ratio of alpha to beta substituents. This isomerization product is recycled to the dealkylation step, with the result that a higher conversion per pass of the alkyl naphthalenes to naphthalene is achieved.

The invention is described more specifically with reference to the accompanying drawing which is a schematic flowsheet illustrating a preferred processing operation for producing naphthalene from a hydrocarbon stock containing alkyl naphthalenes.

The process as illustrated in the drawing involves a preliminary catalytic hydrocracking-desulfurization step adapted to condition the alkyl naphthalene charge material prior to a thermal dealkylation step for producing the naphthalene. The charge, which enters the system through line 10, is a gas oil fraction boiling in the range of 400-550 F. and containing alkyl naphthalenes, and preferably is a catalytic gas oil aromatic extract boiling in the range of 440-515 F. and containing a major proportion of dicyclic aromatics together with a minor proportion of aromatics having only one aromatic ring and only a small amount or no saturated hydrocarbons. For example, a preferred charge may contain 60% dicyclic aromatics, 35 monocyclic aromatics and 5% saturates.

The heated charge together with hydrogen from line 11 passes through line 12 to a catalytic hydrocracker 13 which preferably contains a desulfurization catalyst such as cobalt molybdate on alumina or molybdenum desulfide on alumina. The conditions for conducting this catalytic Conditioning step include a temperature within the range of 800-980o F., a pressure of -1000 p.s.i.g., with a range of 200-500l p.s.i.g. preferred, a hydrogen to hydrocarbon mole ratio of 3:1 to 2521 and preferably 5:1 to 15: l, and a liquid hourly space Velocity of 0.5 to 10 (volumes of charge per hour per bulk volume of catalyst). The hydrogen consumption under these conditions should be between 65-500 s.c.f. per barrel of liquid feed per percent sulfur in the feed and preferably between 200 and 400 s.c.f. per barrel. This Conditioning step effects cracking of most of the saturates and some of the monocyclic aromatics and also converts most of the sulfur in the hydrocarbon stock to hydrogen sulfide.

From hydrocracker 13 the reaction product preferably is sent through line 14 to fractionator 15 from which normally gaseous components are removed overhead through line 17 and a C5-400 F. gasoline fraction is obtained from line 18. The 400+ F. fraction which contains the alkyl naphthalenes is removed via line 16 and sent to thermal dealkylator 21. Alternatively all of the reaction product from hydrocracker 13 can be sent to the dealkylator as indicated by dashed line 27.

The 400+ F. fraction from line 16 is admixed with a recycle fraction from line 33, as hereinafter specified, and the mixture passes through line 20 together with hydrogen introduced via line 19 into dealkylator 21. In the preferred embodiment the dealkylation is effected thermally without a catalyst. The conditions for this operation include a pressure of 150-1000 p.s.i.g., preferably 200-800 p.s.i.g., a hydrogen to hydrocarbon mole ratio within the range of 3:1 to 25 :1 and preferably 5 :1 to

15:1, a residence time of 2-300 seconds with a preferred residence time of 10-60 seconds, and a temperature above 1000 F., preferably within the range of 1200-1400 F., sufficient to efiect dealkylation of alkyl naphthalenes. In this reaction, as previously stated, only a partial dealkylation occurs and the alkyl groups which are in the alpha position on the naphthalene nucleus are removed at about twice the rate as-those in the beta position. Hence the reaction product which leaves the reactor through line 22 contains, in addition to the desired naphthalene, unreacted naphthalenes and partially dealkylated naphthalenes, and the alkyl naphthalene portion of the mixture is enriched with respect to beta alkyl groups as compared with the charge material fed to dealkylator 21.

The reaction product from line 22 passes to fractionator 23 from which gases and a C-400 F. aromatic gasoline cut are removed, respectively, from lines 26 and 25. The desired naphthalene product is taken from line 24 as material boiling in the 400-450 F. range. Typically this fraction is composed predominantly of naphthalene and has a freezing point of 78.6 C. and a sulfur content of the order of 0,06%.

The 450+ F. material withdrawn from fractionator 23 via line 28 is composed mainly of monomethyl and dimethyl naphthalenes with the alkyl groups in the beta position predominating. This stream also contains a small amount of material boiling above dimethylnaphthalenes which desirably should be removed. Hence the stream is passed through line 28 to fractionator 29 from which the monoand dialkyl naphthalenes are obtained overhead through line 30 and the higher boiling material is removed as bottoms via line 32.

In order to increase the yield of naphthalene from the process, the unconverted alkyl naphthalenes should be continuously recycled to the dealkylation zone. However, direct recycling will result in a relatively low overall rate of conversion in the dealkylator due to the high beta alkyl content of this material. Accordingly, the present invention provides a means of increasing the overall rate of the dealkylation reaction by first subjecting the alkyl naphthalenes to catalyic somerizaion in zone 31.

This effects a shift of a substantial portion of the alkyl groups to the alpha position. For example, -methylnaphthalene is partly converted to a-methylnaphthalene. Likewise, 2,3-dimethylnaphtlalene is partly converted to 1,3- and 1,4-dimethylnaphthalene, and 2,6-dimethyl and 2,7-dimethylnaphthalenes are partly converted to alpha methyl naphthalenes such as the 1,5-, 1,6-, 1,7- and 1,8- isomers. It should be noted, however, that the isomerization does not effect any shift of an alkyl group from one of the aryl rings to the other in the same molecule, although some amount of disproportionation between separate molecules may occur.

One manner of effecting the isomerization in zone 31 is to contact the alkyl naphthalenes with any solid acidic cracking catalyst such as silica-alumina, silica-magnesia, silica-zirconia and acid activated clays. The reaction temperature should be in the range of 300-500 C. and more preferably 350-400 C. The liquid space velocity can vary between 0.1 and 20 volume hydrocarbon per volume catalyst per hour and more preferably is maintained in the range of 0.5-6.0. It is desirable to conduct the isomerization at a low hydrocarbon partial pressure and generally in the range of 0.05-0.5 atmosphere, as otherwise coking tends to occur rapidly with resultant deactivation of the catalyst. The low partial pressure can be maintained either by holding a vacuum in isomerization zone 31 or by introducing an inert diluent along With hydrocarbons, for example, nitrogen, hydrogen, methane, propane, butanes, and the like. Whenever the activity of the catalyst has dropped enough to require regeneration, this can be done in conventional manner merely by blowing air through the hot catalyst to burn off the coke deposits. Thereafter the catalyst can be re-used for further isomerization.

By way of example as to the effectiveness of the abovedescribed isomerization procedure in producing alpha substituted naphthalenes, when 2,3-dimethylnaphthalene was reacted over a silica-alumina cracking catalyst at a temperature of 375 C., a liquid hourly space Velocity of about 0.3 and using heptane as diluent so that the partial pressure of the dimethylnaphthalene was 0.046 atmosphere, the isomerization product had the following composition:

Mole percent 1,3-dimethylnaphthalene 43 1,4-dimethylnaphthalene 34 2,3-dimethylnaphthalene 20 1,2-dimethylnaphthalene Negligible Methylnaphthalenes 3 These data show that at least 77% of the 2,3-isomer was converted to products having an alpha substituent.

The isomerization in zone 31 can also be eifected at low temperature using HF-BF3 as catalyst. In practicing the isomerization in this manner, the alkyl naphthalenes from line 30 are first dissolved in a suitable solvent, such as benzene or heptane, and the mixture is contacted with the catalyst at a temperature preferably in the range of 0-30 C. and generally for several hours. Only a small amount of BF3 need be used and the HF can be present in a large molar excess over the BP3 to provide sufficient catalyst volume for good contact. After the isomerization the solvent can be removed by distillation (not shown) and the isomerized product can be recycled through line 33 to dealkylator 21. Generally, some amount of tarry material may be formed during this type of isomerizing operation and it also can be removed by distillation prior to recycling the isomerizate.

By Way of example, when a 10 weight percent solution of 2,3-dimethylnaphthalene in benzene was contacted at 27 C. for 4 hours with BF3 in HF solution, about of the dimethylnaphthalene product was the 1,3-isomer. When 2,7-dimethylnaphthalene was contacted similarly for 20 hours, about 20% of the product Was the 1,7- isomer.

The above-describedV process can be modified by carrying out the dealkylation reaction in zone 21 catalytically employing a desulfurizaton catalyst such as is used in the hydrocracker 13. The presence of the catalyst in the latter step facilitates the dealkylation reaction and in some cases permitsit to be carried out at a lower temperature than that required for thermal dealkylation. The catalyst also effects the conversion of any remaining sulfur into hydrogen sulfide and hence permits the preparation of naphthalene having negligible sulfur content. The conditions for the catalytic dealkylation step include a pressure of -1000 p.s.i.g. with a range of 200-500 p.s.i.g. preferred, a hydrogen to hydrocarbon mole ratio of 5:1 to 25 :1, a liquid hourly space Velocity of 0.2-5.0, and a temperature above 1000 F., usually between 1100 F. and 1200 F., suflicient to dealkylate the alkyl naphthalenes and convert any remaining sulfur mainly into hydrogen sulfide.

In both the hydrocracker and dealkylator of each of the foregoing embodiments, coking occurs after a time to such extent that coke removal from the reaction zone is required. This can be done in conventional manner by passing an oxygen-containing gas through the reaction zone to burn out the Coke. In the steps in which a desulfurizing catalyst is used, burning of coke from the catalyst restores its catalytic activity.

We claim:

1. Process for producing naphthalene from an aromatic hydrocarbon charge stock comprising a mixture of alkyl naphthalenes which comprises subjecting said charge stock in the presence of hydrogen to a temperature in the range of 1000-1500 F., whereby partial dealkylation of alkyl naphthalenes occurs and alkyl groups in the alpha position are removed at a rate faster than alkyl groups in the beta position; distilling the reaction product to obtain a naphthalene fraction and an alkyl naphthalene fraction; subjecting said alkyl naphthalene fraction to isomerizing conditions effective to shift the position of alkyl groups on the naphthalene nucleus, whereby an alkyl naphthalene product having an increased content of alpha alkyl naphthalenes is obtained; and recycling said alkyl naphthalene product to the partial dealkylation step.

2. Process according to claim 1 Wherein said partial dealkylation is elfected thermally Without a Catalyst at a temperature of 1200-1400 F., pressure of 200-800 p.s.i.g., hydrogen to hydro'carbon mole ratio of 3:1 to 2521, and a residence time of 2-300 seconds.

3. Process according to claim 2 wherein said charge stock is an aromatic extract separated from catalytic gas oil.

4. Process according to claim 3 Wherein said charge stock boils within the range of 400-550 F.

5. Process according to claim 1 Wherein said isomerizing conditions include contacting said alkylnaphthalene fraction With a solid acidic catalyst in the presence of an inert diluent at a temperature of 300-500 C., hydro- References Cited by the Examiner UNITED STATES PATENTS 2,617,838 11/52 Nickels 260-668 2,674,635 4/54 Beckberger 260-672 2,700,638 l/55 Friedman 260-672 OTHER REFERENCES Brooks et al: The Chemistry of Petroleum Hydrocarbo=ns, vol. 2, published by Reinhold Pub. Co. (N.Y.), 1955, pp. 127-8 relied upon.

ALPHONSO D. SULLIVAN, Primary Examner.

MILTON STERMAN, Examner. 

1. PROCESS FOR PRODUCING NAPHTHALENE FROM AN AROMATIC HYDRACARBON CHARGE STOCK COMPRISING A MIXTURE OF ALKYL NAPHTHALENES WHICH COMPRISES SUBJECTING SAID CHARGE STOCK IN THE PRESENCE OF HYDROGEN TO A TEMPERATURE IN THE RANGE OF 1000-1500*F., WHEREBY PARTIAL DEALKYLATION OF ALKYL NAPHTHALENES OCCURS AND ALKYL GROUPS IN THE ALPHA POSITION ARE REMOVED AT A RATE FASTER THAN ALKYL GROUPS IN THE BETA POSITION; DISTILLING THE REACTION PRODUCT TO OBRAIN A NAPHTHALENE FRACTION AND AN ALKYL NAPHTHALENE FRACTION; SUBJECTING SAID ALKYL NAPHTHALENE FRACTION TO ISOMERIZING CONDITIONS EFFECTIVE TO SHIFT THE POSITION OF ALKYL GROUPS ON THE NAPHTHALENE NUCLEUS, WHEREBY AN ALKYL NAPHTHALENE PRODUCT HAVING AN INCREASED CONTENT OF ALPHA ALKYL NAPHTHALENES IS OBTAINED; AND RECYCLING SAID ALKYL NAPHTHALENE PRODUCIT TO THE PARTIAL DEALKYLATION STEP. 